A warning has been issued to farmers after an inquest into the death of a man whose body was found beneath a layer of tarpaulin in a silage pit.

Stephen Victor McMinn (25), from Tirkelly Road in Rathfriland, went missing while helping his uncle and cousins build a silage clamp at their farm on the evening of May 16, 2014. He was later found trapped under a layer of plastic sheeting.

It is believed that the welder succumbed to the effects of carbon monoxide and nitrogen dioxide within seconds, leaving him unable to escape from beneath the tarpaulin.

It is the first death of its kind to happen in Northern Ireland since records began in 1968.

A Craigavon Court inquest into Mr McMinn's death was told that the group had laid tyres on top of the lower plastic sheeting layer to stop it blowing away.

After laying the next layer, Stephen's uncle, Robin McMinn, and cousin, Richard Crawford, crawled beneath it to retrieve the tyres.

Mr McMinn went missing as multiple layers of sheeting were being laid on the pile.

His disappearance was not immediately noticed because he had told his cousins he was leaving to meet his girlfriend.

It was only when his wallet and keys were found inside his car at the farmyard that a search was launched.

Mr McMinn was found lying on his side close to the edge of the sheeting at the top of the silo. Subsequent attempts to revive him were unsuccessful.

Coroner Suzanne Anderson said she would be writing to the Agriculture Minister, the Ulster Farmers' Union and the Health and Safety Executive (HSENI) to raise awareness of the dangers of working with silage.

While the HSENI has updated its guidance on working with silage since the tragedy, Coroner Anderson said: "I think this is something I need to raise formally with them and make sure that any education that can be conducted for the farming community is carried out."

The HSENI's Malcolm Downey said the silo was built according to common practice.

"I would estimate that in the last 40 years, close to two million silos here have been covered in this way," he added.

"At some stage, Stephen entered the area below the silage cover and was overcome by the severe lack of oxygen and tragically died as a result."

He also explained that since its investigation, the HSENI has worked with Stephen's parents to adapt the guidance in Great Britain on working with silage to Northern Ireland, with a section warning not to go between the layers added to the advice.

Carbon monoxide levels as high as 870 parts per million were recorded in the part of the clamp where Stephen was found in the wake of the tragedy, according to commander Edward Carroll, hazardous materials protection officer with NI Fire and Rescue Service.

Mr Carroll said it was unusual to find such high levels of carbon monoxide in such a case, but that was not to say that it did not happen.

"I've never seen such high levels in 26 years in the Fire and Rescue Service," he added.

The deputy state pathologist for Northern Ireland, Dr Alastair Bentley, who spoke to the inquest by video link, said he believed Stephen could have lost consciousness within seconds.

"It is a case of asphyxiation due to entrapment beneath a tarpaulin in a silo," he added.

Stephen's mother, Louise McMinn, said her son had been planning to meet his girlfriend, Rachel Mayne, at 9pm, and had gone to help his uncle and cousins with the silage at 6.30pm. His final text to his girlfriend was sent at 8.45pm.

Robin McMinn told the inquest his son, Kyle, had put his foot through the sheeting where a tyre was sitting beneath, so Robin had gone under the sheets to retrieve the tyre.

He said that after the group finished laying the plastic, they went in for their tea. While they noticed Stephen's car was still there, they assumed he had been collected.

The coroner told the court: "I am extending my deepest sympathy to Stephen's family circle. This is a terrible tragedy for the entire McMinn family."